1. Field of the Invention
The present invention relates to a flow velocity meter, more particularly, to a flow velocity meter capable of transmitting continuous waves with specific frequencies for measuring a speed of a current according to a frequency difference between a transmitted wave and a reflected wave.
2. Description of the Prior Art
A flow velocity meter is used for measuring a speed of a current under test. For instance, it can calculate the speed of current in a period of time is after measuring the speed of current in a river or stream and further judge if damage occurs in river downstream or if a reservoir at river downstream necessarily sluices.
Acoustic Doppler Velocimeter (ADV) is the most known flow velocity meter in the prior art besides of velocity measuring devices with mechanical vortex valve. The ADV measures a flow speed of river or fluids at a single point in two dimensions or three dimensions by three transducers in array. The transducers in the middle transmit a high power pulse wave along the axis of the device, and the three transducers in array receive.
According to Doppler principle, the transducers receive pulse waves in rivers or in a reservoir, transmitted in directions and reflected by suspended particles in fluids. Then a Doppler radar determines moving direction of the particles according to a change in audio and calculates the speed of the particles by Doppler principle. Assuming that the particles move with the current, the flowing direction of the particles is considered as the flowing direction of the current.
A supersonic Doppler radar, however, requires accurate installation for calculating velocity components in different dimensions, and the supersonic Doppler radar reflects all suspended particles in fluids. Therefore, the system will misjudge if there are a huge number of impurities in water. Besides that, the supersonic Doppler radar does not fit for measuring a speed of a current in river and reservoirs far away.
Therefore, microwave Doppler radar is invented. Compared with the supersonic Doppler radar using pulse wave to measure, the microwave Doppler radar measures a speed of a current in a long term to calculate an average of a speed of a current by using continuous waves or frequency modulated continuous waves. In hence, the microwave Doppler radar does not need accurate installation and analysis of a speed of a current. A general measuring formula of a microwave Doppler radar is that:
                              F          d                =                              2            ⁢                          v              ·              cos                        ⁢                                                  ⁢            θ                    λ                                    (        1        )            
where ν is a relative speed of a current for a radar, θ is an angle between a velocity vector in a direction of a current and a line of radar centers, and λ is a wavelength of center frequency from an output signal (radio-frequency signal in general)
However, there are also faults for a microwave Doppler radar.
First, according to the formula (1), Doppler shift Fd is directly relative to the center frequency of a radio-frequency signal (RF signal), and the center frequency is easily changed with temperature and time to drift the center frequency of the RF signal and further an error in Doppler shift.
Furthermore, an error in the angle between the velocity vector in a direction of a current and a line of radar centers is attributed to inaccurate installation angle of the microwave Doppler radar. For instance, an antenna pattern is vertical to the surface of the water if θ is 90 degree. It is known that the velocity component is 0 according to the formula. In addition, the angle θ is correspondent to 90 degree in the condition of the largest velocity component. If so, the microwave Doppler radar has to install on the surface of the water (surface of fluids)
Therefore, the industry has to develop a new flow velocity meter to solve the above problems.